Electric vehicles, hybrid vehicles, fuel-cell vehicles, etc. are currently in the development or in the practical implementation stage. These vehicles employ high-output AC rotary electric machines as primary means for generating vehicle driving power, but such high-output AC rotary electric machines have had the problem that their magnetic noise (magnetic sound) is large. Furthermore, quiet AC motors are needed in various applications, and a variety of techniques have been proposed for reduction of magnetic noise of AC rotary electric machines. For example, Japanese Patent No. 2928594 proposes that voltage components that cancel out the harmonics contained in the induced voltage of an electric motor be superimposed on the output voltage of an inverter, thereby bringing the waveform of the electric current to the motor closer to a sinusoidal wave and thus achieving a reduction in torque pulsation and its associated noise.
On the other hand, Japanese Unexamined Patent Publication No. 11-55986, for example, proposes to reduce the torque pulsation of an electric motor by deliberately superimposing a harmonic component (hereinafter sometimes referred to as the harmonic current) on the fundamental frequency component of the electric current fed to the motor.
Further, Japanese Unexamined Patent Publication No. H04-200294 and Japanese Examined Patent Publication No. H07-89753 disclose that the carrier frequency of a PWM control inverter is altered in a prescribed pattern to alter the electromagnetic sound of a motor.